Lasers are known to be important tools for processing a wide range of materials. In particular, lasers are very well suited to and see wide application for processing of metals, polymers, ceramics, semiconductors, composites and biological tissue. By focusing a laser beam, it can be possible to achieve improved precision of the laser's action in a direction transverse to the beam axis. However, localizing the laser's action in the axial direction of the beam can be difficult. During processes such as laser welding, a phase change region (PCR) is created where the material localized to the bonding region changes dynamically from solid to a liquid and/or a gas state and back to a solid again at the completion of the weld. In some cases the material may change multiple times between the various states and also interact with other substances present in the weld zone including other solids, liquids and gasses. Controlling this phase change region (PCR) is important to control the quality of the weld and the overall productivity of the welding system. The high spatial coherence of laser light allows good transverse control of the welding energy deposition, but thermal diffusion limits the achievable aspect ratio of welded features when the energy is transmitted through the material with conduction alone. For higher aspect ratio features, the more dynamic and unstable process of keyhole welding is used to allow the conversion of optical to thermal energy to occur deeper in the material. Here, axial control (depth of the PCR) is even more problematic. In keyhole welding, the depth of the PCR and the absorption of the laser may extend deep into the material (for example, depths from 10 micrometers to tens of millimeters). Here, the beam intensity is sufficient to melt the surface to open a vapor channel (also known as a capillary or “the keyhole”) which allows the optical beam to penetrate deep into the material. Depending on the specific application, the keyhole may be narrow (e.g., less than 1 mm) but several millimeters deep and sustained with the application of optical power (for example in the range from 1-2 W to 20,000 W or more). As a result, the light-matter interaction region inside the PCR can be turbulent, unstable and highly stochastic. Unfortunately, instability of keyhole formation can lead to internal voids and high weld porosity resulting in weld failure, with potential catastrophic consequences. Similarly, keyhole instability can result in spatter that contaminates nearby system components, complicating the application of laser welding in systems such as vehicular transmissions. Weld quality verification is usually required, often using expensive ex-situ and destructive testing. Welding imaging solutions are offered but are limited in their capabilities and usually monitor regions either before or after the PCR, to track the weld joint, or record the top surface of the cooled weld joint.